1. Field of the Invention
This invention relates to improvements in furnaces for baking carbon anodes used in an electrolytic reduction cell. More particularly, this invention relates to an improved baking furnace characterized by a plurality of improved insulated exhaust port plugs therein.
2. Description of the Related Art
Carbon anodes used in electrolytic reduction cells, such as the Hall cells used for the reduction of molten alumina to aluminum, are prepared by baking in a furnace commonly referred to as a ring furnace. Such a furnace is described in Racunas et al U.S. Pat. No. 3,975,149, assigned to an assignee of this invention and cross-reference to which is hereby made.
Such furnaces typically comprise a number of adjacent sections, varying from 60 to 96, each of which may, in turn, comprise 6 to 9 pits in which the anodes to be baked are placed. Each pit has endwalls made of refractory material and sidewalls comprising hollow refractory flues through which hot gases are passed to heat and bake the anodes in the pits. These flues are serially connected together from one pit to the next to permit flow of hot gases from the hollow flues comprising the sidewalls of one pit to the corresponding sidewalls in an adjacent pit. Each flue is provided with a cover or top having one or more ports therein through which intake air, fuel or exhaust gases may pass, as the case may be.
The ports leading to the respective flues are normally closed by the insertion of a removable insulated plug therein. When anodes are to be baked in a particular pit or series of adjacent pits, the covers to the corresponding flues are removed and heating means are inserted in the openings. Such heating means may include a device for feeding fuel into the flue such as described in Racunas et al U.S. Pat. No. 4,025,287, assigned to an assignee of this invention and cross-reference to which is hereby made.
To maximize the efficiency of the furnace, it is customary to bake anodes in a one or more pits in a series and then, while the anodes are cooling, to bake anodes in adjoining pits. Combustion air is then brought in through the uncovered ports in the flues of pits in which the baked anodes are cooling to permit preheating of the air before burning by circulating air over the previously baked, but still hot, anodes. The exhaust gases, in turn, pass out of the flues of the pits in which the anodes are baking and are customarily circulated through adjoining flues of pits in which anodes to be baked have been placed. In this way the heat of previously baked anodes is used to preheat the air used in combustion and the hot exhaust gases are used to preheat anodes yet to be baked.
It is customary to bake the anodes for an extended period of time of about 30 to 35 hours. When the baking period for each pit or group of pits is over, the firing means are moved to the next series of ports which may comprise ports which previously served as the exhaust ports. Correspondingly, the previous air intake ports are closed by the insertion of cover plugs, and the ports previously occupied by the firing means become the air intake ports as the anodes in these pits cool. Plugs covering the ports of flues in the next set of pits are then removed to serve as the new exhaust ports. This procedure continues to be repeated as the baking furnace operates on a continuous basis.
Over an extended period of operation, of necessity, a number of plugs must be removed or inserted to respectively uncover or cover ports to the respective flues as the firing progresses from pit to pit or section to section, as the case may be. Since the plugs must be able to withstand exposure to combustion temperatures, the plugs must comprise a refractory material capable of withstanding such temperatures. Solid refractory materials, e.g., a ceramic plug, would be satisfactory from a standpoint of temperature but would result in an unacceptable weight in view of the frequent and multiplied insertions and removals of the plugs.
It has, therefore, been customary to utilize a plug made from ceramic fibers held together in an organic binder. Unfortunately, during use, such organic binders eventually burn away and the plug must be replaced.
It would, therefore, be desirable to have a baking furnace with removable plugs which, while retaining the lightweight characteristics of prior art plugs, would be better able to withstand the heat to which the plug will be exposed during operation of the furnace.